The project is designed to investigate the processes involved in assembling and maintaining cellular membranes. We use as a model system the protozoan Tetrahymena pyriformis. This single-celled organism is typical of aimal cells in having the usual variety of functionally different organelles, e.g. nucleus, mitochondria, lysosomes, etc. Our study encompasses the biosynthesis and metabolic interrelationships existing among the membranes of these different cell parts. Radioisotopes are used to analyze rates of lipid and protein synthesis and deployment from their biosynthetic sites to their structural destinations. The functionally distinct organelles have membranes differing quite markedly in phospholipid composition as well as in protein makeup. We have determined some of the factors important in bringing about the lipid differences. The most valuable attribute of Tetrahymena as a model system for these studies is the ease with which they can be manipulated. Very significant differences in lipid and protein metabolism can be detected in response to a number of environmental factors, including temperature changes, starvation, and addition of inert solid particles. Striking changes in membrane metabolism also occur at certain periods during the cell cycle, as analyzed using synchronously dividing cells. The ability to evoke responses of this type provide us with the opportunity to study the regulation of membrane growth and turnover.